American Medical Association Family Medical Guide - American Medical Association [277]
Prenatal Testing
A number of pregnancy tests and procedures are available to help doctors evaluate the health of the pregnancy and the fetus. Some of the tests, such as amniocentesis, can determine if a fetus has a genetic abnormality or a malformation. Other tests, such as ultrasound, can evaluate the development of the fetus, detect any structural abnormalities, and date the pregnancy. For various reasons, pregnant women may choose not to have the tests. For example, some women would never consider terminating a pregnancy even if a severe or lethal defect were found in the fetus. During your pregnancy, discuss your feelings with your doctor, who can help you evaluate the benefits and risks of having the tests.
Ultrasound
Ultrasound (see page 111) is an imaging procedure that uses sound waves to produce pictures of the developing fetus on a monitor. Ultrasound does not involve radiation or X-rays and is harmless, painless, and noninvasive. Ultrasound is offered to most women at least once during their pregnancy and more frequently to women who may be at risk of having problems. During the first trimester, ultrasound is often used to confirm and date a pregnancy by determining the size and shape of the fetus. Many doctors recommend an ultrasound between the 17th and 19th weeks of pregnancy to evaluate fetal growth and development and to rule out some structural abnormalities such as neural tube defects (see page 398) or cleft lip or cleft palate (see page 403). Ultrasound can also detect twins and, after the 16th week, can sometimes determine a fetus’s gender. During late pregnancy, ultrasound may be used to evaluate the health, size, and position of the fetus and the placenta in preparation for delivery.
Rh Disease in Pregnancy
When a woman with Rh-negative blood has a fetus with Rh-positive blood, some of the fetus’s blood may enter the woman’s bloodstream at birth, or during pregnancy. If the woman is not given Rh immunoglobulin within 72 hours of delivery, her immune system will develop antibodies to fight the Rh-positive blood, which it recognizes as foreign. These antibodies don’t cause problems in a first Rh-incompatible pregnancy, but if the woman does not receive the Rh immunoglobulin and has a subsequent Rh-positive fetus, her antibodies could cross the placenta and destroy the fetus’s red blood cells, possibly causing anemia or even death. After a miscarriage or the termination of a pregnancy, Rh immunoglobulin should always be administered to an Rh-negative woman.
Abdominal ultrasound
In an abdominal ultrasound, a device called a transducer is moved along the surface of the pregnant woman′s abdomen, emitting sound waves that reflect off the fetus and the woman’s internal organs. The sound waves are electronically translated into an image on a computer screen.
Vaginal ultrasound
In a vaginal ultrasound, a narrow transducer (called a vaginal wand) is inserted into the vagina to produce images on a monitor. Vaginal ultrasounds may be used in the first trimester of pregnancy for a number of reasons, such as to see a fetus more clearly or to confirm (or rule out) a pregnancy or an abnormality such as an elongated cervix (which can indicate an impending miscarriage) or an ectopic pregnancy.
AFP and Triple Screen Blood Tests
All women are offered one of two blood tests between the 15th and 20th weeks of pregnancy to measure the level of alpha-fetoprotein (more familiarly known simply as AFP), a protein produced by the developing fetus. AFP is present in amniotic fluid and, in smaller amounts, in a pregnant woman’s blood. One test measures just AFP. The triple screen blood test measures AFP and two hormones, human chorionic gonadotropin (HCG) and estriol (a form of estrogen), which are produced by the placenta during pregnancy. The interpretation of these measurements is based on factors such as a woman’s age, race, and weight.